An optical glass of high-index and low-dispersion properties is useful for favorably correcting aberration in an optical system. Such an optical glass is therefore used in the optical system (the lens) of an optical instrument. The optical glass of high-index and low-dispersion properties is set forth, for example, in Japanese Patent Kokai No. 2007-8782. The optical glass set forth in Japanese Patent Kokai No. 2007-8782 includes SiO2, B2O3, and La2O3 as a basic composition.
As an example of the optical instrument, a fluorescence microscope is cited. In the fluorescence microscope, the observation and measurement of a specimen are carried out by fluorescent light generated from the specimen. In recent years, the fluorescence microscope has come to be used for an observation with fainter fluorescent light, for example, a single-molecule fluorescence observation, and the fluorescence intensity measurement of a specimen with low fluorescence intensity. Such an application needs the observation of a fluorescent image of higher contrast or a high degree of measurement accuracy.
Here, in the observation of the fluorescent image (a specimen image) and the measurement of the amount of fluorescent light, auto-fluorescence from glass used for the lens of the optical system is known as one of causes of the deterioration of image contrast and measurement accuracy.
The auto-fluorescence refers to fluorescent light emanating from glass. The auto-fluorescence is generated by the fact that when excitation light exciting the specimen passes through the lens, part of the excitation light is absorbed into the glass of lens material. The wavelength of the auto-fluorescence sometimes nearly coincides with that of the fluorescent light emanating from the specimen. In this case, if the intensity of the auto-fluorescence is greater than that of the fluorescent light emanating from the specimen, the information of a dark portion (a portion in which the fluorescence intensity is low) of the fluorescent image will be lost, for instance.
In this way, the auto-fluorescence reduces the contrast of the fluorescent image of the specimen. As a result, in an observation apparatus in which the auto-fluorescence is generated, a good observation of the fluorescent image becomes difficult. Moreover, in the fluorescence intensity measurement, the auto-fluorescence becomes a background noise. Consequently, in a measurement apparatus in which the auto-fluorescence is generated, it becomes difficult to make a measurement with a higher degree of accuracy in the specimen of a low fluorescence intensity.